1. Field of the Invention
The present invention relates to a braking control system for a motor vehicle. More particularly, it relates to braking control technology in which the pressure increase of a hydraulic pressure control module for controlling the braking force of each wheel is appropriately decided or judged.
2. Description of the Prior Art
There have heretofore been known vehicular braking control systems each of which is furnished with automatic braking control means for automatically controlling the braking forces of individual wheels, utilizing wheel speeds obtained from wheel speed sensors mounted on the respective wheels.
In executing such a braking control for a motor vehicle, inevitably both the vehicle speed (the ground speed) of the motor vehicle and the wheel speeds mentioned above need to be detected at high precisions by reason that the increases or decreases of the pressures of the hydraulic pressure control module for controlling the braking forces of the wheels are decided or judged in accordance with the lock or slip states of the wheels.
By way of example, the official gazette of Japanese Patent Application Publication No. 7231/1975 discloses the vehicular braking control system in which the vehicle speed is estimated from the maximum one of the wheel speeds of the four wheels, whereupon the braking forces of the wheels are controlled on the basis of the comparisons between the estimated vehicle speed and the wheel speeds of the respective wheels.
In the prior art, whether or not the pressure is to be increased is decided or judged by checking theslip state in view of the difference between the estimated vehicle speed and each wheel speed. In this regard, the wheel speed sometimes rises or falls suddenly due to the change of a road surface condition. However, since the vehicle speed does not change so suddenly as the wheel speed, the estimated vehicle speed which is based on only the information of the maximum wheel speed can deviate greatly from the actual vehicle speed of the motor vehicle. In order to avoid this drawback, the vehicle speed is estimated with a predetermined guard value taken into account for the maximum wheel speed. The guard value is set at a comparatively large value in consideration of a low-.mu. road (where symbol .mu. denotes a friction factor).
The above contrivance, however, is not satisfactory yet. More specifically, in such a case where, even during its drive on a high-.mu. road, the motor vehicle descends along a drop in road level or passes through a very-low-.mu. road part, the wheel cannot receive the reaction force of the surface of the road, and the wheel speed falls once. Thereafter, when the wheel comes to receive the reaction force of the road surface on a high-.mu. part again, abrupt rise in the wheel speed occurs temporarily. In the case of estimating the vehicle speed from the maximum wheel speed, therefore, the estimated vehicle speed sometimes becomes apparently higher than the actual vehicle speed.
In a case where such a situation is to be coped with in accordance with the idea of the guard value in the prior art, it might be thought out to set the guard value small. The guard value in the prior art, however, has the basic concept that "the actual vehicle speed can never rise abruptly in excess of the guard value". Accordingly, it is naturally limited to lower the guard value. That is, in the light of the aim thereof, the guard value in the prior art is not set much lower than the value to which the vehicle speed may possibly rise in actuality.
Meanwhile, even when the wheel speed has changed due to the temporary change of the road surface condition, actually the vehicle speed hardly changes in many cases. With the guard value, accordingly, it is not in rare cases that the vehicle speed is estimated somewhat higher than in actuality.
On this occasion, in the case where the slip state is judged from the difference between the estimated vehicle speed and each wheel speed, the vehicular braking control system incurs the problem that, in correspondence with the higher estimative component of the vehicle speed, the pressure increase is not judged necessary in spite of being actually required, so the operation of increasing the pressure cannot be started (the braking force cannot be intensified).